Perfluoropolyethers having brominated end groups and fluoroelastomers obtained therefrom

ABSTRACT

New perfluoropolyethers comprising repeating units chosen from among ##STR1## statistically distributed along the chain, having a controlled molecular weight and brominated end groups, prepared starting from the product of the photochemical oxidation of C 2  F 4 , and/or C 3  F 6 , containing peroxidic oxygen, and subsequent treatment in liquid phase with bromine, at temperatures ranging from -40° to 130° C. in the presence of ultraviolet radiations, said new perfluoropolyethers derivatives being useful as additives in rubber mixes vulcanizable with peroxides, in amounts of 0.01 to 5 parts by w per 100 parts by weight of elastomer, optionally in combination with non brominated perfluoropolyethers, to improve the processability in the extrusion and the detachability of the vulcanized articles from the molding.

This is a divisional of application Ser. No. 07/251,356 filed Sep. 30,1988 which in turn is a combination of application Ser. No. 832,735,filed Feb. 25, 1986 and Ser. No. 917,550 filed Oct. 10, 1986.

BACKGROUND OF THE INVENTION

This is a combination of prior application Ser. No. 832,735 filed Feb.25, 1988 by Giuseppe Marchionni and Gian Tommaso Viola and applicationSer. No. 917,550 filed Oct. 10, 1986 by Giulio Tommasi, Raffaele Ferroand Gianna Cirillo.

This invention relates to new perfluoropolyethers having a Br atom boundto one or both the end groups of the chain.

More in particular this invention relates to new mono- or bifunctionalperfluoropolyethers having a controlled molecular weight and consistingof units chosen from amongst ##STR2## statistically distributed alongthe perfluoropolyether chain.

A further object of the present invention is that of providing a processfor preparing the abovesaid perfluoropolyethers with brominated endgroups and having a controlled molecular weight by a method which iseasily practicable on a commercial scale and is capable of providinghigh yields of brominated product.

It is known that the reaction of oxygen with tetrafluoroethylene and/orperfluoropropene, carried out at a low temperature in the presence of UVradiations and in an inert solution, generally a fully fluorinatedcompound or a chlorofluorinated compound, leads to a perfluoroetherproduct having a peroxide oxygen content which varies as a function ofthe operative conditions under which the photooxidation has beeneffected.

The process and the peroxidic products are disclosed for example inBritish Patent Nos. 1,226,566 and 1,104,482.

The thermal decomposition of the peroxide, which is carried out toremote peroxide bridges containing neutral end groups of the type --CF₃,CF₂ CF₃ and acylic end groups of the type --OCF₂ COF.

Actually, the thermal decomposition reaction of the peroxide product forproviding the neutral perfluoropolyether gives rise to radicals of thetype R_(f) O--CF₂ (RF being a perfluoroether chain), which react withone another, thus leading always to perfluoropolyethers having a highand non-controlled molecular weight.

The decomposition of the peroxidized precursors is usually carried outby a heat treatment carried out in a wide temperature range, generallyfrom about 100° C. up to high temperatures, preferably from 180° to 250°C.

DETAILED DESCRIPTION OF THE INVENTION

It has now surprisingly been found that it is possible to prepare newmono- or bifunctional perfluoropolyethers with brominated end groups anda controlled molecular weight and with high yields if the treatment ofthe peroxidized precursor, having a proper peroxide oxygen content as afunction of the desired molecular weight of the final product, isconducted under suitable conditions.

The new perfluoropolyethers with brominated end groups according to theinvention have the general formula: ##STR3## wherein m, n, p and r areintegers from 0 to 50, m+n+p+r being at least 2, and wherein A and B,equal or different between them, are end groups chosen from the class:##STR4## wherein X is Br or F, and at least one of the groups A and Bcontains a bromine atom.

The perfluoro-oxyalkylene units of general formula (I) are statisticallydistributed along the perfluoropolyether chain.

The process according to the invention for preparing mono- orbifunctional perfluoropolyethers with high yields consists in subjectinga peroxidized perfluoropolyether, obtained from the photooxidation oftetrafluoroethylene and/or perfluoropropene, having a predeterminedperoxide oxygen content, to photolysis with UV radiations, in liquidphase, in the presence of bromine at temperatures ranging from -40° to130° C., either or not in the presence of a completely fluorinated orchlorinated inert solvent, the liquid reaction medium being fullysaturated with bromine.

The product resulting from the photooxidation, which contains peroxidebridges, can be utilized as such if the peroxide oxygen content isalready at the value suitable for obtained the desired molecular weightof the final brominated perfluoropolyether.

Should the peroxidized precursor have a higher peroxide oxygen contentthan desired, then it is subjected to thermal treatments according toconventional techniques, such as the ones described in theabove-mentioned British patent.

The term peroxide oxygen content (P.O.) means the amount in grams ofactivated oxygen referred to 100 grams of perfluoropolyether.

The average molecular weight of the brominated perfluoropolyether isdirectly related to the peroxide oxygen content of the startingperfluoropolyether.

In fact in correspondence of the peroxidic bridges, during thebromination according the invention, occurs a cleavage of theperfluoropolyether chain: consequently the reduction of averagemolecular weight is proportional to the peroxidic bridges in the chain.

The solvent utilized in the bromination reaction is any fullyfluorinated or, as an alternative, chlorofluorinated compound, which isinsert in the reaction conditions and does not contain unsaturations.

As solvents useful to the purpose there may be cited, for example, theperfluorocarbon or the fluorocarbons.

To prepare bromine-saturated solutions, it is generally operated havingthe bromine present as a bottom body in the reaction apparatus.

The preferred reaction temperature is in the range of from 60° to 120°C., more preferably from 90° to 110° C.

In fact, by operating under the above-cited conditions it is possible toobtain very high yields of brominated perfluoropolyethers; therefore itcould be assumed that the radicals, which form from the peroxidedecomposition, completely react with bromine, providing only end groupsof type --CF₂ Br, or ##STR5## If there is only one peroxidic bridge inthe starting perfluoropolyether chain, the obtained products shall beprevailing by monobrominated compound. If there are many peroxidicbridges in the starting perfluoropolyether chain, the obtained productsshall be prevailing formed by dibrominated compounds.

If it is operated at higher temperatures than 130° C., theperfluoropolyethers according to the invention are still obtained,however, they contain only very low percentages of brominated inparticular of dibrominated perfluoropolyethers.

By the process of the present invention it is possible to prepare bothhigh molecular weight perfluorobrominated compounds, depending on theperoxide oxygen content of the starting precursor, or by acting on thetemperature at which the photolysis of the initial peroxide is carriedout.

In fact, the high molecular weights are obtainable by operating attemperatures from -40° to 80° C., while low molecular weight brominatedperfluoropolyethers are obtainable in a temperature range of from 80° to130° C.

The brominated products according to the invention can be obtained alsoby carrying out the bromination in the high case however the yield ofbrominated perfluoropolyether is very low and such a process could nothave any interest from a technical point of view.

Furthermore it has been found that by the simple heat treatment it isnot possible to obtain perfluoropolyethers with a controlled molecularweight depending on the peroxide content of the startingperfluoropolyether.

The process indicated hereinbefore permits to overcome all the abovesaiddrawbacks. In fact the new brominated perfluoropolyethers of theinvention are obtained by an utmostly simplified process, with very highyields and with a molecular weight regulated as a function of theperoxide content of the starting peroxidized perfluoropolyether. Afurther purpose of the present invention is to provide rubbers which arevulcanizable by peroxides and exhibit improved processability propertiessuch as a better extrudability and a better detachability of thevulcanized article from the molding dies utilized in injection moldingor compression molding technologies.

The invention is based on the use of additives of the class ofperfluoropolyethers and derivatives thereof having brominated endgroups, comprising the following formulas: ##STR6## in which: T=F or CF₃; m, n and q are integers, zero included, m+n+q ranges from 2 to 200;##EQU1## is 50 when n and/or q are different from zero; when m=0, q/n is≧0.2 and ≦20 when n is other than zero; when q=0, m/n ranges from 1 to50 when n is other than zero; A and B, either like each other ordifferent from each other, are selected from the groups ##STR7## X beingbromine or fluorine; when m is different from zero, X can be bromineonly in A or in B.

Particularly suitable are the liquid products with a mean molecularweight ranging from 500 to 10,000, the preferred being those whichcontain at least one bromine atom in at least one of the end groups Aand B.

Perfluoropolyethers of formula:

    F--(CF.sub.2 CF.sub.2 CF.sub.2 --O).sub.n --CF.sub.2 CF.sub.3 (II)

where n is an integer greater than 2 and lower than 200, preparedaccording to the method described in European patent No. 148,482, mayalso be used.

It is possible also to use mixture of neutral compounds of formula (I)and/or formula (II) with the brominated compounds of formula (I). Forneutral compounds it is intended a perfluoropolyether having terminalgroups without bromine.

Products which are suited as additives according to the invention andcomprised in formula (I) are the perfluoropolyethers prepared byphotochemical oxidation of C₂ F₄ or C₃ F₆, commercially known asFlombin^(R) Z and Flombin^(R) Y and the perfluoropolyethers of the classof the polymers derived from perfluoropropene-epoxide, commerciallyknown as Krytox^(R). It is possible to utilize also perfluoropolyethersprepared by photochemical oxidation of C₂ F₄ or C₃ F₆.

Particularly suitable have proved to be brominated perfluoropolyetherderivatives prepared according to the process described in Italianpatent applications No. 19653 A/85 and No. 22517 A/85, and obtained asmixture of products at different degrees of bromination.

The additives according to the present invention are dispersed by mixingin rubbers vulcanizable by peroxides or by mixed peroxy and ionicvulcanizing systems or in mixture thereof, to improve theirprocessability.

As is well known, vulcanizing systems based on peroxides are capable ofvulcanizing many types of rubbers, imparting to the vulcanized articlesgood properties with respect to stability to heat, to chemical agents,and to mechanical stresses.

The rubber mixed vulcanizable with peroxides, per se known in the art,generally consist of the following components:

RUBBER

As types of elastomers vulcanizable with peroxides there may be cited:natural rubber, ethylene/propylene/diene copolymers, butadiene/styrenerubbers, butadiene/acrylonitrile rubbers, silicone rubbers,fluoroelastomers containing peroxy vulcanization sites, mixtures ofrubbers of which at least one is vulcanizable with peroxides or withmixed vulcanization systems, and in particular mixtures of afluoroelastomer based on CH₂ =CF₂ and of an elastomeric polymer C₂ F₄/C₃ H₆, of the types of ALFAS^(R).

PEROXY STARTER (vulcanizing agent)

This consists of an organic peroxide, such as for example:benzoylperoxide, dicumyl peroxide,2,5-dimethyl-2,5-di-(t.butylperoxy)-hexane, α,α'-bis(t.butylperoxy)-diisopropykbenzene, aliphatic or cyclicbis-peroxycarbonates.

VULCANIZING COAGENT

This consists of an unsaturated di- or tri-functional compound such ase.g., triallylcyanurate, triallylisocyanurate, divinylbenzene,m-phenylene-bis(maleimide).

REINFORCING FILLERS

Carbon black, silica, etc.

ACID ACCEPTORS

Such as e.g. the oxides of lead, zinc, calcium, magnesium.

PROCESSING COADJUVANTS

These coadjuvants are products endowed with plasticizing or lubricatingproperties, such as e.g., vegetable waxes, low molecular weightpolyethylene, various stearates, polyesters, octadecylamines, and thelike.

The use of such coadjuvants becomes necessary owing to the poorextrudability characteristics of the mixtures and to the phenomena ofadhesion to the dies and soiling of them.

The conventional processing coadjuvants give rise, in a few cases, toproblems of interference with the peroxy vulcanization system, and of anon-satisfactory appearance of the surface of the molded piece.Furthermore, in the specific case of the fluoroelastomers, a "thermalstabilization" (post-vulcanization) at temperatures of from 200° to 250°C. for a time ranging from 10 to 30 hours is absolutely necessary toimpart to the fluoroelastomers the desired properties of stability withrespect to heat, to chemical agents, and to solvents. During thisoperation, the common plasticizing agents (glycols, stearates, organicphosphates, oleates, phthalates etc.) volatilize and/or decompose.

This causes a remarkable degradation of the mechanical properties of thefinal article, in particular:

an excessive increase in hardness;

a decay of the stability to compression set;

a decay of the elastic characteristics (low elongation value and highmodulus value);

a low thermal stability which precludes the use thereof in severeapplicative conditions (180° C.).

In conclusion, the advantages achievable during the processing result inunacceptable shortcomings in the final article.

Thus, the use of plasticizing or lubricating agents is limited to smallamounts (1-3 p.h.r.), i.e., parts per hour of rubber, which do notsensibly alter the final properties of the vulcanized article, but, onthe hand, are insufficient to meet the aforesaid requirements.

In particular, it has been ascertained that the utilization ofperfluorosilicone oils permits the hardness to be lowered by 4-5 points,but exhibits however some difficulties as regards the admixing to andthe incorporation into the fluoroelastomer. Furthermore, a reduction inthe vulcanization rate and a worsening of the compression set value andthermal stability value can be observed.

The use of fluorinated polymers having a very low molecular weight andthe consistency of waxes, such as Viton LM produced by Du Pont, wasadopted to impart better rheological characteristics to the mixtures,and in particular during extrusion. In this way it is possible to obtainmanufactured articles of large size and with complicated profiles.However, the use of such a "plasticizer" in amounts of 10-20 parts byweight for 100 parts of fluoroelastomer is possible only withconventional vulcanizing systems, while it is not consistent withvulcanizing systems based on peroxides. The result thereof is aworsening of the stability with respect to heat, to chemical agents, tosolvents and oils, and of the compression set value.

Surprisingly, by using the additives according to the present inventionin amounts below 1 part by weight when the additive is neutral, andranging from 0.01 to 5 parts by weight per 100 parts of rubber when theadditive is brominated, and preferably up to 3 parts by weight in thecase of additives having a mean molecular weight equal to or higher than2,500, and up to 1.5 parts by weight in the case of additives having amean molecular weight below 2,500, mixes were obtained--the otherformulation elements remaining the same (rubber, starter, crosslinkingco-agent, acidity acceptors)--exhibited a better behavior as regardsboth processability by extrusion and the detaching of the vulcanizedarticle from the dies.

The use of high molecular weight additives improves in particular theprocessability in terms of yield and appearance of the extrudate, whilethe lower molecular weight additives more easily migrate to the surface,thus imparting in particular an excellent detachability of thevulcanized article from the molding die. For this reason, the amount ofthese lower molecular weight additives are to be limited to preventgreasiness phenomena or, in the case of a high bromine functionality, asensible interference with the vulcanization process.

In fact, if it is operated with the proper amounts of additivesaccording to this invention, these additives do not sensibly interferewith the vulcanizing system: actually, vulcanization tests carried outaccording to standard ASTM D 2084 with an oscillating-disc rheometer(Monsanto), did not show significant differences with regard either tothe rate or the vulcanization yield. Conversely there was observed alowering of the minimum torque, which means a better processability ofthe mixture.

Furthermore, the bromine-containing additive chemically binds to theelastomeric chain during the vulcanization process.

In fact, if the mixture containing the additive is subjected toextraction tests with CFCl₂ --CF₂ Cl as solvent, the additive isrecovered in the extraction liquid. Conversely, if theadditive-containing mixture is vulcanized, a subsequent extraction withthe same solvent does not lead to the recovery in the liquid of theadditive.

This, in practice, leads to a further improvement: actually, the finalarticle retains in the long run an excellent surface appearance and doesnot exhibit greasiness phenomena.

The use of the additives according to the present invention turns out tobe particularly suitable for the fluoroelastomers in general, such asfor example the copolymers of CH₂ =CF₂ with C₃ F₆ or with C₃ F₆ +C₂ F₄and containing peroxy crosslinking sites, the copolymers of C₂ F₄ withpropylene or with perfluoroalkyl-perfluorovinyl ethers (in particularmethylvinyl-ether) containing, in the latter case, a cure site monomer,terpolymers of C₃ F₆, vinylidene fluoride andperfluoroalkyl-perfluorovinyl-ether (in particular methylvinylether)containing a cure site monomer.

EXAMPLES

The following examples are given merely to illustrate the presentinvention, without limiting, however, the scope and applicativemodalities thereof.

A PREPARATION OF BROMINATED PERFLUOROPOLYETHERS EXAMPLE 1

Into a cylindrical photochemical reactor having a maximum capacity of300 ml, with an optical path of 0.5 cm, equipped with a coaxial quartzsheath for housing a mercury vapor lamp type Hanau TQ 150 or a noble gasdischarge lamp, for example a Xenon lamp type PEK Inc. X-75, equippedwith a magnetic stirrer, a reflux dropping device, a CO₂ trap and athermoregulation system for both the reactor and the sheath system,there were introduced 430 g of perfluoropolyether obtained fromtetrafluoroethylene having a peroxidic oxygen (P.O.) content of 1.04% byweight, a viscosity of 9,500 cSt (at 20° C.), an average molecularweight of 39,930 and a m/n ratio, determined through NMR (¹⁹ F)analysis, equal to 0.8. 5 ml of bromine were added, the whole was mixedand the temperature was brought to about 100° C. Subsequently, afterswitching on of the lamp, 20 ml of bromine were added during the test,which lasted 14 hours. At the conclusion of the test, the reaction masswas discharged into a flask and bromine was distilled under vacuum.

Obtained were 390 g of a product having a viscosity of 20 cSt andpractically free from P.O., and having average molecular weight (PM) of3550, such product exhibiting, on NMR (¹⁹ F) analysis, the followingstructure:

    BrF.sub.2 C--O(C.sub.2 F.sub.4 O).sub.m (CF.sub.2 O).sub.n --CF.sub.2 Br

with a m/n ratio=0.8, analogous with the one of the starting product.

The chemical shifts of the brominated end groups (δ, ppm; CFCl₃) were,respectively:

    ______________________________________                                        --OCF.sub.2 OCF.sub.2 Br                                                                            -19.9 ppm                                               --OCF.sub.2 CF.sub.2 OCF.sub.2 Br                                                                   -18.4 ppm                                               ______________________________________                                    

The product yield in brominated products was equal to 96% with respectto 4% of neutral product of formula:

    RO--(C.sub.2 F.sub.4 O).sub.n --(CF.sub.2 O)--R'

where R and R', equal or different from each other, may be --CF₃ and--CF₂ CF₃.

EXAMPLE 2

Into the photochemical reactor of example 1 there were introduced 125.3g of perfluoropolyether obtained from tetrafluoroethylene having a P.O.of 1.04% by weight and a viscosity of 9500 cSt, along with 280 g of thethroughly fluorinated solvent (1,2-perfluorodimethyl cyclobutane.

After mixing of the mass, 2 ml of bromine were added and the temperaturewas brought to 0° C.

Subsequently, after switching on of the lamp, further 6 ml of brominewere added during the test which lasted 20 hours. At the conclusion ofthe test the reaction mass was discharged into a flask and the bromineas well as the solvent were distilled under vacuum.

Obtained were 109 g of product having a viscosity of 32 cSt and beingpractically free from P.O., having PM of 4630, such product exhibiting,on NMR (¹⁹ F) analysis, the same structure as the dibrominated productof example 1.

EXAMPLE 3

Into a 200-cc flask equipped with a stirrer, a thermometer, a cooler anda dropping device, 100 g of perfluoropolyether obtained fromtetrafluoroethylene, having a P.O. equal to 1.2% by weight, viscosity of5300 and PM of 31,300 were introduced. The reactor temperature wasbrought to 150° C. and 8 ml of bromine were added at regular intervalsduring 5 hours; subsequently the temperature was brought to 210° C. andthe mixture was refluxed during additional 5 hours. At the end of thetest, bromine was removed under vacuum, so obtaining 79 g of a producthaving a viscosity of 17 cSt, and PM of 4,300 which, on NMR (¹⁹ F)analysis, revealed to have practically the same structure as thedibrominated product obtained in example 1, but with a yield equal to 5%with respect to 95% of completely fluorinated neutral product.

EXAMPLE 4

120 g of perfluoropolyether of example 3 obtained fromtetrafluoroethylene, having a P.O. equal to 1.2% by weight, wereintroduced into the flask of example 3. The reactor temperature wasbrought to 150° C. and 8 ml of Br² were added during the 5-hour test;subsequently the temperature was raised to 180° C. and the whole wasallowed to react during additional 5 hours. At the conclusion of thetest, bromine was removed under vacuum and 93 g of a product having aviscosity of 21 cSt were obtained; such product, subjected to NMR (¹⁹ F)analysis, proved to have practically the same structure as thedibrominated product obtained in example 1, but with a yield equal to4.5% with respect to 95.5% of completely fluorinated neutral product.

EXAMPLE 5

420 g of perfluoropolyether obtained from tetrafluoroethylene with aP.O. of 1.5% by weight, a viscosity of 14400 cSt and a molecular weightof 46,200 were introduced into a photochemical reactor similar to theone described in example 1.

After switching on of the lamp and maintaining a temperature of 100°, 25ml of bromine were added during 18 hours. At the conclusion of the testthere were obtained 345 g of a product having a viscosity of 7 cSt andPM of 2,600 with a yield of dibrominated product equal to 96.5% besides3.5% of neutral product.

EXAMPLE 6

Under the same conditions of example 5 and using the sameperfluoropolyether, but at a temperature of 60° C., there were obtained350 g of a product having a viscosity of 45 cSt and an average molecularweight of 5,250.

The % amount of dibrominated product was higher than 85%.

EXAMPLE 7

Under the same conditions and using the same perfluoropolyether ofexample 5, but operating at a temperature of 120° C. there were obtained330 g of a perfluoropolyether with a viscosity of 10 cSt and a molecularweight of 3,000; in this case the dibrominated product amount was equalto 74%.

EXAMPLE 8

A peroxide perfluoropolyether obtained by photochemical oxidation oftetrafluoroethylene, having a mean molecular weight of 24,500 and aperoxide content equal to 0.9% by weight, was reduced by a thermaltreatment at a temperature of 160° C. during 5 hours until obtaining aproduct having an average molecular weight of 18,000 and a peroxideoxygen content equal to 0.35% by weight.

400 g of such product were places into a photochemical reactor similarto the one described in example 1 and were irradiated in the presence ofBr₂ (7 ml) during 10 hours at a temperature of 100° C.

After removal of Br₂ under vacuum, there were obtained 380 g of aproduct having a viscosity of 65 cSt and a content of products withbrominated functionalities equal to 95% by weight.

EXAMPLE 19

Into the photochemical reactor of example 1 there were introduced g 400of perfluoropolyether obtained from C₃ F₆, having a peroxidic oxygencontent (P.O.) of 0.7% by wt and a molecular weight of 2,550 (osmometricmeasure).

Subsequently 10 g bromine are added and the temperature is raised to100° C. After switching on the lamp further bromine was added during thetest (20 g during 15 hours). At the end of the test the reaction masswas discharged into a flask and residual bromine was distilled undervacuum.

The obtained product (395 g) showed an average molecular weight of1,220, was practically free from P.O. and contained brominated andgroups --CF₂ Br and ##STR8## in amount equal amount equal to abrominated end group for each perfluoropolyether chain, the other endgroup being of the type: ##STR9##

EXAMPLE 10

Into a photochemical reactor having capacity of 1,000 ml and on opticalpath of 2 cm equipped with coaxial quart sheats for housing a mercuryvapor lamp type Marian TQ 150, with traps and thermoregulation systemsuitable for maintaining the temperature at -40° C., were introduced1000 g of perfluoropropene. After switching on the lamp, a mixture O₂+C₂ F₄ in molar ratio 5:1 was fed the total amount of gases being fed ata rate of 192 l/h (measured at atmospheric pressure). After 150' thelamp was switched off and 380 g of a perfluoropolyether having P.O.equal to 3.23% and viscosity of 1064 cSt (20° C.).

From the NMR analysis it cam out that the structure consists of C₃ F₆ Ounits randomly alternated with CF₂ CF₂ O and CF₂ O units and ofperoxidic units.

The reduction of the peroxidic oxygen content was obtained by subjectingthe product to the irradiation of a U.V. lamp in the reactor used inexample 1 and maintaining the temperature at 0° C. After 18 hours theP.O. content was 1.1% and the molecular weight of the product was 2,780.

In the same reactor was carried out the bromination reaction at 100° C.,by introducing 10 g bromine at the beginning and then further 40 gduring the following 30 hours. Peroxidic oxygen was completelyeliminated.

After removal of excess bromine, 340 g of perfluoropolyether wereobtained, of average molecular weight 1050, having brominated end groups##STR10## near neutral end groups of the type --OCF₃ and acid end groupof the type ##STR11##

The functionality of the product, expressed as ratio between brominatedend groups and number of perfluoropolyether chains is 1.3.

USE AS ADDITIVES FOR FLUOROELASTOMERS EXAMPLES 11-20

By the usual mixing techniques there were prepared rubber mixturescomprising the ingredients indicated in Table. With the mixes siprepared, the tests and determinations indicated in said Table 1 werecarried out.

Ingredients used in the mixture

Elastomer 1: CH₂ =CF₂ /C₃ F₆ /C₂ F₄ terpolymer in the molar ratio 2/1/1containing as a cure site monomer a brominated olefin, having a Mooneyviscosity ML (1+4) at 100° C.=104 and a total bromine content of 3,500ppm (VITON®GF).

Elastomer 2: CH₂ =CF₂ /C₃ F₆ copolymer in a molar ratio 3.5/1 andcontaining as a cure site monomer a brominated olefin, having a Mooneyviscosity ML (1+4) at 100° C.=109 and a total bromine content of 2,000ppm (FLUOREL®2480).

Elastomer 3: C₂ F₄ /C₃ H₆ copolymer in a molar ratio 1/1, having aMooney viscosity ML (1+4) at 100° C.=130 (AFLAS®). Luperco® 101XL=2,5-dimethyl-2,5-di(terbutylperoxy--hexane: product at 45% by weight,with 55% of inert filler.

TAIC: triallylisocyanurate.

Black MT: carbon black for rubbers (ASTM N 990)

Additive 1: Perfluoropolyether from hexafluoropropene, brominated at theend group, containing on the average about 1 bromine atom per molecule(functionality: about 1), having a viscosity of 30 cSt at 20° C. and amolecular weight of about 1,220.

Additive 1 was prepared as follows: into a cylindrical photochemicalreactor having an optical path of 0.5 cm, equipped with a coaxial quartzsheath for housing a mercury vapor lamp, type Hanau TQ150) or a noblegas (for example Xenon) discharge lamp, type PEK, Inc×75, equipped witha magnetic stirrer, a reflux condenser, a CO₂ trap, and athermoregulation system for both the reactor and the sheath system,there were charged 400 g of perfluoropolyether from C₃ F₆ having anactivated oxygen content equal to 0.75% by weight, and a molecularweight, osmometrically determined, equal to 2,550 u.m.a., i.e., atomicmass units.

10 g of bromine were added and the temperature was brought to 100° C.Subsequently, after lighting of the lamp there were added 20 g ofbromine during the test, which lasted overall 15 hours. At the end ofthe preparation, the reaction mass was discharged into a flask and theresidual bromine was distilled under vacuum.

Obtained were 395 g of a product having a mean molecular weight equal to1,220 u.m.a., practically no oxidation power, and having brominated endgroups of the type OCF₂ Br and ##STR12## in the ratio of one brominatedend group for each perfluoropolyether chain, the other end group beingcomposed of trifluoromethyl units --O--CF₃ or acyl fluorides ##STR13##or formyl fluoride ##STR14##

Additive 2: Perfluoropolyether from brominated tetrafluoroethylene witha functionality degree in bromine of about 1.8, having a viscosity of29.8 cSt at 20° C. and a molecular weight of about 4,500.

Additive 2 was prepared as follows: into a cylindrical photochemicalreactor having a capacity of 300 cc, with an optical path of 0.5 cm,equipped with a coaxial quartz sheath for housing a mercury vapor lamp,type hanau TQ 150 or a noble gas discharge lamp, for example Xenon, oftype PEK, INC X 75, equipped with a magnetic stirrer, a reflux droppingfunnel, a CO₂ trap, and a thermoregulation system both for the reactorand for the sheath system, there were charged 420 g ofperfluoropolyether from tetrafluoroethylene having a peroxy oxygencontent (P.O.) of 1.1% by weight, a viscosity of 10,800 cSt (at 20° C.)and an m/n ratio, determined by NMR analysis (19F), equal to 0.95. Afteraddition of 5 ml of Br₂, the whole was mixed and the temperature wasbrought to 110° C. and maintained at this value for 16 hours, in thecourse of which a further 25 ml of bromine were added. At the end of thereaction, after removal of the bromine there were obtained 397 g of aproduct having a viscosity of 29.8 cSt (at 20° C.) and a molecularweight, osmometrically determined, equal to 1.8.

Additive 3: perfluopropolyether from neutral tetrafluoroethylene havinga viscosity of 30 cSt.

The Tests Performed

Detachability test: the mixture was vulcanized in a press at 170° C. for15 minutes, the test piece was them removed from the mold at 170° C. andthe detachability was immediately evaluated on the basis of thepercentage of breaks and/or of residue of vulcanized product, if any,adhering to the mold. In Table 1 the detachability is indicated by:

S (low) if, out of 4 test pieces, all test pieces break and/or leaveresidues of vulcanized product on the mold; or 1 test piece at the mostdetaches without breaking and/or leaving residues of vulcanized producton the mold;

D (fairly good) if, out of 4 test pieces, 2 or 3 test pieces detachwithout breaking and/or leaving residues of vulcanized product adheringto the mold;

B (good) if, out of 4 test pieces, all test pieces detach withoutbreaking and/or leaving vulcanized residues adhering to the mold.

Processability test in extruder

The processability was evaluated according to method ASTM D2230/78,which classifies the extrudates both as the extrudate amount in gramsper minute (Method B) and as the surface appearance of the Garvey rating(Method A - System B).

Characteristics of the vulcanized test obtained

The characteristics were determined on test pieces obtained byvulcanization in an oven at 250° C. for 16 hours, the postvulcanizationbeing preceded by a temperature rise from 100° C. to 250° C. in 8 hours.

In Table 1, examples 1, 7 and 9 are comparative examples, namely:example 1 is to be compared with example 8; example 9 is to be comparedwith example 10.

From the comparison between the examples of the invention with theadditive and those without additive with regard to the thermal stabilitycharacteristics, it is not possible to recognize a sure effect of theadditive according to the invention, as the variations occurring uponvariation of the additive fall within the variability of the measuringmethod.

                                      TABLE 1                                     __________________________________________________________________________    EXAMPLE          11  12  13  14  15  16  17  18  19  20                       __________________________________________________________________________    ELASTOMER 1 (b. weight)                                                                        100 100 100 100 100 100 --  --  --  --                       ELASTOMER 2 (b. weight)                                                                        --  --  --  --  --  --  100 100 --  --                       ELASTOMER 3 (b. weight)                                                                        --  --  --  --  --  --  --  --  100 100                      LUPERCO 101% L ® (b. weight)                                                               3   3   3   3   3   3   3   3   3   3                        TA10 (b. weight) 3   3   3   3   3   3   3   3   3   3                        P60 (b. weight)  3   3   3   3   3   3   3   3   3   3                        BLACK MT (b. weight)                                                                           30  30  30  30  30  30  30  30  30  30                       ADDITIV 1 (b. weight)                                                                          --  0.5 --  1.5 3   --  --  3   --  --                       ADDITIV 2 (b. weight)                                                                          --  --  --  --  --  3   --  --  --  3                        ADDITIV 3 (b. weight)                                                                          --  --  0.5 --  --  --  --  --  --  --                       THERMOMECHANICAL                                                              CHARACTERISTICS                                                               ODR at 180° C. (1)                                                     Min. torque (inch. lbs)                                                                        22  22  22  21  20  20  25  24  17  16                       ts. 10 (sec.)    79  81  70  81  84  80  84  87  109 111                      ts. 50 (sec.)    114 120 115 123 132 123 117 133 211 295                      Max. torque (inch. lbs)                                                                        115 108 109 105 98  97  112 98  75  68                       PROCESSABILITY (2)                                                            yield (a) (g/min)                                                                              32  32  32  39  45  49  44  47  26  35                       Garvey rating (b)                                                                              6   8   8   9   10  10  9   11  8   8                        DETACHABILITY    S   D   S   D   S   D   S   D   S   D                        VULCANIZED TEST-PIECE                                                                          6.1 6.2 6.2 6.1 5.0 4.9 4.0 3.1 4.3 3.9                      CHARACTERISTICS                                                               M 100 (MPa)                                                                   (modulus at 100) CR                                                           (MPa) (tessile stress)                                                                         19.4                                                                              21.6                                                                              21.5                                                                              20.3                                                                              17.0                                                                              17.0                                                                              17.0                                                                              14.8                                                                              16.6                                                                              16.6                     AD (%)           216 224 216 216 219 220 278 292 261 261                      (elongation at break)                                                         Hardness, Shore A                                                                              72  75  75  73  70  71  70  70  70  70                       (points) (4)                                                                  COMPRESSION SET  33  33  34  33  33  33  32  32  48  48                       at 200° C. × 70 h (6)                                            O-ring (%) 25 × 3.53                                                    (in mm)                                                                       CHEMICAL STABILITY IN                                                         BP Olex (MK 4409) at                                                          150° C. for 7 days (6)                                                 P (%)            0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 3.3 3.3                      V (%)            0.8 0.6 0.8 0.8 0.8 0.8 0.8 0.8 6.0 6.0                      TERMAL STABILITY                                                              at 275° C. for 70 h (7)                                                CR (%)           -62 -43 -44 -36 -60 -62 -55 -36 -44 -40                      AR (%)           +44 +41 +31 +35 +59 +57 -34 -34 +16  +1                      Hardness (points)                                                                               -1  -2  -2  -2  -1  -1  -1  -2  -7  -5                      __________________________________________________________________________     (1) ASTM D2084                                                                (2) ASTM D2230-78 (a) Method B, (b) Method ASystem B                          (3) ASTM D412                                                                 (4) ASTM D2240                                                                (5) ASTM D395 Method B                                                        (6) ASTM D471                                                                 (7) ASTM D573                                                            

We claim:
 1. Perfluoropolyethers of the formula: ##STR15## wherein m, n,r and p are integers from 0 to 50, m+n+4+p being at least 2, and whereinA and B, equal to or different from each other, are end groups selectedfrom the group consisting of: ##STR16## wherein X is fluorine orbromine, at least one of the end groups A and B containing a bromineatoms, the perfluoro oxyalkylene units being distributed randomly alongthe perfluoropolyether chain; and further characterized in that they areprepared by starting from peroxyperfluoropolyethers obtained throughphotochemical oxidation of C₂ F₄ and/or C₃ F₆.